Nowadays, magnetostrictive multilayer systems, such as spin-valve systems, are used in a plurality of applications. One field of application that has increased in importance recently comprises the usage of magnetostrictive GMR/TMR resistor structures as pressure sensors.
Pressure sensors operate generally according to the principle of deflection of moveable membranes or other moveable elements, by pressure impact, wherein the membrane deflection represents a measure for the applied pressure. Thereby, the membranes of the pressure sensors can be realized in silicon technology via bulk micromechanic (BMM) or in surface micromechanic (SMM). Pressure sensors in surface micromechanic measure, for example, a change of capacitance between membrane and substrate against electrode induced by the membrane deflection. Thereby, the capacitance swings are so small, that significant effort has to be made in signal processing, which results in an increased chip area. Further, the increased signal processing effort leads to additional cost increase of production, since additional effort is required for realizing the corresponding circuits.
In pressure sensors in BMM technology, membrane deflection is detected via the piezoresistive effect, wherein the expansion of the membrane is evaluated via expansion-induced change of resistance, for example at certain doped areas. However, the so-called gauge factor, which corresponds to quotients dR/R/expansion, i.e., which is a measure for sensitivity, is only small for silicon with about 40, which results in a reduced signal noise ratio.
Due to its higher gauge factor, which can be up to about 600 for a TMR structure, the approach of GMR/TMR resistor structures promises in comparison to the piezoresistive effect, improved sensitivity, higher signal/noise ratio as well as pressure measurement with increased resolution.
However, in many applications it is required to connect two or more such sensor elements to a bridge arrangement, for example a Wheatstone bridge, to obtain a signal indicating the generated expansion or the applied pressure, respectively.
Normally, for realizing a sensor bridge, a full bridge is used, wherein the signal, i.e. a change of resistance, is oppositely rectified in both branches. Normally, this is obtained by oppositely magnetizing the reference layer, which requires additional effort, for example by requiring meandering conductive trace foil. The additional effort also increases the production cost of the sensors, where it would be desirable to provide a sensor with lower production cost, due to the numerous applications for which pressure sensors are suitable, and due to the fierce competition.
General basics of multilayer sensors can be found in the references Löhndorf et al “Highly sensitive strain sensors on magnetic tunneling junctions”, Appl. Phys. Lett., Vol. 81, pp. 313-315, Löhndorf et. al. Strain sensors based on magnetostrictive GMR/TMR structures”, IEEE Trans. Magn, Vol. 28, pp. 2826-2828, September 2002, Ludwig et al, “Adapting GMR sensors for integrated devices”, Sensors and Actuators A, 106, pp. 15-18, 2003.